Depends. I have seen signs in private/commercial campgrounds that instruct people to pay the next morning. In public places like state parks and U.S. Forest Service campgrounds, there are often fee envelopes and an "Iron Ranger," which is basically a safe. You are supposed to pick a campsite, fill out the envelope and deposit the fee. You usually tear off a tag from the envelope that you then hang on a post at your site as proof that you paid. There is a chance you could get rousted if you don't comply. For example, park police may patrol the facility. If they see you occupying a site without the tag they might think you are trying to camp without paying. Some state and federal facilities have campground hosts. They are volunteers who live at the facility for some or all of the season. One of their duties is to check to make sure people have properly registered. I try to make sure I have plenty of $1 bills on me if I am planning to camp at federal facilities as the fees can often be odd amounts, like $7. Don't want to be stuck with only $20 bills.